Milky quartz veins are visible throughout all parts of Scottsdale's McDowell Sonoran Preserve (the Preserve) (see locator map below) and at many scales. Prospectors searching for heavy metal and mineral deposits have historically been attracted to milky quartz and represent the earliest researchers. Pits and tailings are occasionally found with milky quartz, however written records of mining activity are usually not found.
Quartz veins and large milky quartz deposits are commonly found in Proterozoic rocks throughout Arizona. Research targeted at milky quartz has not been done in the greater Phoenix area, and as such, little is known about their formation and emplacement history. Thus, previous works in the region and by topic were researched during our study. Karlstrom and others (1991) provided valuable context to the Fraesfield site. Jahns (1953) provided great insight into pegmatites in the White Picacho District, as they relate to pegmatites at Paraiso. Several other topical references, cited throughout this report, were used to help characterize both sites.
Early geologic mapping in the Fraesfield and Paraiso areas have generally noted the presence of milky quartz in association with its host rock and identified larger milky quartz bodies. Detailed geologic mapping in the area and region have provided important context and geologic setting crucial for studying quartz veins. Christensen and others (1978) conducted the first detailed map of the southern half of the McDowell Mountains. Péwé and others later mapped portions of northern Scottsdale and the Ft. McDowell area in 1983 (published 2012). Skotnicki and others mapped most portions of the Preserve area (Skotnicki, 1996; Skotnicki and others, 1997). Vance (2012) conducted detailed structural analysis and mapping of the McDowell Mountains as part of his MS thesis at Arizona State University. And most recently Steve Skotnicki has completed the most detailed and comprehensive mapping of the McDowell Mountains and its foothills (Skotnicki, 2014, unpublished).

The southwestern portion of the McDowell Mountains, located in the McDowell Sonoran Preserve in Scottsdale, Arizona, was last mapped geologically more than 30 years ago. The curiosity of volunteers with the McDowell Sonoran Conservancy—a non-profit organization created to advocate for, educate about, and provide stewardship to the Preserve—about rocks from a prehistoric tool quarry in that area led to a project to conduct a preliminary geologic survey of the vicinity near the quarry.
This project was conducted largely by volunteers who spent hundreds of hours working in the field with permission and managerial oversight from the City of Scottsdale Preservation staff and under the scientific supervision of Brian F. Gootee, research geologist with the Arizona Geological Survey. The volunteers did most of the field data collection, recording, and preliminary analysis, much of the mapping, and some initial geologic interpretation. Gootee did the thin-section analysis and photomicrographs, most of the geologic interpretation, and conducted numerous field visits and reviews of the volunteers’ work.
Because of the unusually extensive volunteer involvement in the project, this paper describes the project approach, process, and intermediate findings of the field work and analysis in addition to the final project results. Each phase of the project is described based on the then-available information, analysis, and interim hypotheses. As more information was collected and more analysis was performed, the conclusions and therefore the nomenclature changed. This narrative approach accurately reflects the nature of field work and the scientific process.
The investigation area was approximately rectangular around the tool quarry, located near a prominent ridge called Lost Dog Overlook in the southwestern foothills of the McDowell Mountains. The mountains and surrounding areas, including the investigation area, are part of the McDowell Sonoran Preserve. The investigation focused on the tool quarry area in order to determine why the rocks in that particular area were useful for tool-making.
The rocks in the vicinity of the tool quarry reflect an extensive series of geologic events in several major episodes over the last 1,700 million years. A series of eruptions approximately 1,700 million years ago produced rhyolite flows. Subsequent burial, deformation, metamorphism, mineralization, uplift, and exposure produced an array of mineralized colors, fractures, joints, shear zones, and erosional patterns now visible at the surface. In addition, the survey noted newly-discovered outcrops of granite and limestone within the investigation area.

Publication Date

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